The invention relates to a circuit arrangement according to the preamble of claim 1, for controlling light emitting diodes (LED), which are combined in groups including at least one driver which has outputs for supplying electric current to the LED's and a control input for adjusting the electric current supplied to the LED's.
Such a circuit arrangement is well-known in the state of the art and is used for example for controlling LED's of a billboard or indicator board. As a driver, by which for example a group of sixteen LED's is energized or, respectively, supplied with electric current, a multitude of different types is available. All have in common that the current supplied to the LED's is adjustable by means of a resistor which is connected to the control input.
When the light output of an LED group, that is, the light output of the LED's or the LED group controlled by the driver is to be changed, the resistance must be changed. To this end, the resistor may be in the form of a potentiometer.
Since the light output of the LED groups often needs to be adjusted to the ambient of the LED group, a corresponding adjustment of the resistor is necessary. This however is quite costly, if a multitude of LED groups is combined to an indicator board.
A big problem in the adjustment of the light output of a LED group (or respectively, the output current of the respective driver unit), for example, of an indicator board resides in the fact that one has to be within reach of the resistor or respectively the driver and, consequently, the LED group so that the light output change cannot be satisfactorily monitored. When, for example, the light output of a LED group is to be changed in order to adjust the light output of a new LED group to the light output of the adjacent. LED groups as it is necessary for example upon the replacement of a defective LED group by a new LED group, it is generally necessary that, after a change of the resistor setting the repair person moves away from the indicator board to be able to better judge the brightness of the new LED group in relation to the brightness of the adjacent LED groups. If the brightness is still not correct the repair person has to return to the indicator board to change the resistance value and again has to move away from the indicator board for judgment. This needs to be repeated until the brightness is correct. This procedure however is very unsatisfactory.
A further problem with the known control of LED's resides in the fact that the brightness of the LED's cannot be changed continually during operation. It is, for example, not possible so far, to adjust the brightness of the LED's during operation to the ambient light conditions without problems. The LED's of an indicator board have for example the same brightness in daylight with the sun shining as they have at night when the moon is shining.
It is the object of the present invention to provide a circuit arrangement for controlling light emitting diodes in such a way that the brightness of an LED group is more easily adjustable and that it can also be changed during normal operation in a simple way.